Six-year-old Katelynn Drydale's body grows like other girls' her age -- mostly at night, a few centimeters per year. Except for her right leg. Every six months or so, it grows one and a half to two centimeters in the course of a few minutes.

The Bonner Springs Elementary first-grader's right knee and about half of her right femur have been replaced with a prosthesis featuring Star Trek-style technology. With the use of electromagnetic pulses, the artificial "bone" can be lengthened without invasive surgery to keep up with the growth of her other leg. The prosthesis is known as a Repiphysis implant, and in Katelynn's case it connects to her tibia below the knee and to the upper half of her femur.

In November 2004 Katelynn was diagnosed with osteogenic sarcoma, a rare bone cancer that produces malignant bone growth. Katelynn's orthopedic oncologist, Dr. Howard Rosenthal, director of the Mid-America Sarcoma Institute at Menorah Medical Center in Overland Park, estimated 450 children a year are diagnosed with the disease in the United States.

In March of 2005, Rosenthal operated on Katelynn to remove the malignant bone mass and all the tissue it touched. In the same surgery, he implanted the prosthesis. Rosenthal said without this prosthesis, Katelynn would have had to undergo invasive surgery every six months in order to implant successively larger prostheses to keep up with the growth of her other leg.

Rosenthal said that with 10 years and several feet of growth before her, that would have meant "a lot of school to miss" for Katelynn.

Rosenthal said the other best option for Katelynn would have been a kind of double amputation -- joining her tibia to her femur and rotating her foot 180 degrees -- that would facilitate her using an external prosthesis.

To explain how the prosthesis expansion works, Rosenthal said it was like a toilet paper roll holder, with one tube fitting over another and a spring inside the larger tube.

The compressed spring is embedded in plastic, and within that plastic is also a receiver. The receiver, activated by the signal from the electromagnetic "wand," heats up the plastic, freeing the spring to slowly expand, which in turn expands the implant.

It requires no anesthetic, Rosenthal said, while the old method did, and required the patient to use crutches for three months.

Rosenthal said Katelynn was the fourth patient he'd implanted a Repiphysis prosthesis in, and that he'd implanted two more since then.

Patrick Fisher is the director of hip marketing for Wright Medical Technologies, the company that made Katelynn's prosthesis. He said 234 had been implanted since the device was approved for sale by the Food and Drug Administration in late 2002.

While the cost for the custom-made prostheses runs upwards of $25,000, Fisher said, it is much less expensive for use in children than a traditional kind that would have to be replaced periodically through costly surgeries.

The device is mostly cobalt chrome, with some titanium, and a special plastic surrounding the metal parts.

Katelynn finished her chemotherapy around Halloween and her hair has begun to grow in. Her mother, Patricia Drydale, said Katelynn was sick of being mistaken for a boy, and so had taken to putting on makeup, including lipstick and eye shadow, for play with her friends.

Her favorite television show is "That's So Raven," because the main character, she said, "is cute."

Katelynn said she planned to grow her hair out and then cut it short.

In her combined kindergarten and first-grade class at Bonner Springs Elementary, Katelynn is treated like just another student -- one who happens to use a walker for distances longer than a few yards.

Although it's been nearly 11 months since she had the surgery, Katelynn still has a difficult time walking, apparently due to scar tissue formed in her knee around the prosthesis. Because of that, her right knee is limited to between 20 and 40 degrees of comfortable movement, her mother said.

During a recent visit to her class by the Chieftain, her classmates anxiously waved their raised hands to tell what they liked about Katelynn and how they understood the reason she used a walker.

Grace Weaver said of the period when Katelynn was undergoing chemotherapy, "She was sick and could get sick real easily."

Her best friend, Paige Krone, said "She likes to make noises and have a lot of fun."

She added, "Sometimes she just likes to go down the slide, then when she comes down she swings on her walker."

Returning from their reading group, Katelynn grinned mischievously after tagging Cameron Davis' toe with her walker on her way out the door, just after he'd warned her not to.

Cameron quipped, "She likes to mess with me."

Katelynn's walker resembles a scaled down version of the kind most often used by the elderly. But hers is purple, and unlike probably most other walker users, she can pull herself up and swing her feet onto the handle.

Katelynn said her favorite part of the school day is "snackin' and lunch."

And although she chose yellow to color a drawing of mittens her teacher instructed the class to use their favorite color on, Katelynn said her "really favorite color" is red.

Her teacher, Karen Marshall, said Katelynn was "a very good reader" and was "doing really good with addition, especially for how much school she's missed."

Katelynn began the school year late because of her chemotherapy and an infection she contracted in her leg, which required her to be hospitalized Aug. 29. She got out Nov. 3.

Katelynn attends physical therapy sessions three times a week at Children's Mercy Hospital in Kansas City, Mo. There, she does a variety of exercises to help her regain the movement in her right knee.

The exercises are intended to be fun, said Katelynn's physical therapist, Tyler Selby.

In a January session, he had her ride a tricycle down a hallway as he timed her and throw a small basketball into a kid-sized basketball hoop while standing with her leg on a wood step.

"The games are meant to get her mind off it (the pain)," Selby said, "yet still get done what we want."

For another exercise, Katelynn had to step over a miniature hockey stick propped on small pylons before picking up various toys.

Selby tried to encourage Katelynn throughout the exercises without being soft on her. "Get that booty down," Selby said at one point as she bent down to pick up a toy.

Several pickings-up later, Katelynn knocked the stick off.

"My leg's getting tired," she said.

Patricia Drydale said Katelynn was "usually sore a little after each session for an hour, and doesn't want to do much."

In early March, when her peers are still looking forward to spring break, Katelynn will be enjoying a vacation with her mother -- a week in Disneyland. The trip is courtesy of the Dream Factory, a philanthropic organization whose 30 offices include one in Overland Park.

Patricia Drydale said she and a friend had submitted Katelynn's name to the agency, whose stated goal on its Web site is to "to provide dreams to children with life-altering illnesses."

Katelynn said she looked forward to "having breakfast with the princesses," but more than that, "I actually want to have ice cream for breakfast."